I. Field of the Invention
The present invention relates to position insensitive shock sensors, and more particularly to position insensitive shock sensors using conductive liquids to complete an electric circuit. The shock sensor has particular utility in such diverse fields as auto air bag trigger circuits, emergency location transmitters (ELTs) for locating aircraft crash sites, and military ordnance fuses, for example.
II. Description of Related Art
There has been a variety of shock or acceleration sensors developed over the years which can be roughly divided into three categories. A first category typically involves a spring suspended weight. The weight may be suspended by a cantilever or a coil spring and when the system undergoes a rapid acceleration or deceleration (i.e. shock) the inertia of the weight causes the weight to be displaced in a direction counter to the force of the spring. When the shock exceeds a threshold level, the weight is displaced in the direction opposite the acceleration or deceleration and causes an electrical circuit to be completed.
A second category of shock sensors involves magnetically suspended weights. These types of shock sensors may involve weights which are magnetically attracted to a fixed magnet or might involve a moveable magnet and a fixed magnet wherein the like poles of the moveable magnet and the fixed magnet are adjacently positioned so that the magnets normally repel one another. When the shock sensors of the second category are subjected to a sufficiently large shock, the inertia of the weight overcomes the magnetic forces to move a fixed distance in order to cause an electrical contact closure for completing an electrical circuit.
A third category of shock sensors involves the use of an electrically conductive fluid as the weight. The conductive fluid typically comprises mercury but could alternatively comprise fine iron filings, conductive materials suspended in fluids, etc. In one type of conductive liquid shock sensor a mercury reservoir is enclosed in a glass tube, and wherein electrical contacts are adjacently disposed at one end of the glass tube. Among the disadvantages of that construction are its glass construction, which is susceptible to fracture, and the variability of friction between the mercury and the side walls, making the operation of the switch difficult to predict. Additionally, gas trapped between the mercury mass and the electrical contacts impedes the motion of the mercury, resulting in a high operating force for the switch. One further disadvantage is that the design constitutes a latching switch which requires that the switch be reset by applying a shock in the opposite direction of its normal operation.
An adjustable mercury switch has been proposed wherein the glass tube is tiltable relative to the anticipated direction of acceleration or deceleration. As the switch undergoes a shock, the mercury climbs uphill to close the electrical contacts. Although the threshold acceleration force required to operate the switch is easily adjusted by simply adjusting the relative tilt of the switch relative to the direction of the force, the switch is highly sensitive to a change in position, i.e., when the orientation of the overall system relative to the direction of the gravitational force is frequently varied, as when a shock sensor switch is installed in a car driven in a hilly environment. A .+-.5.degree. change in angle can cause a .+-.25% change in the operating "G" level point.
A "G" level is defined as the dimensionless ratio of acceleration to the gravitational acceleration. Since m=w/g, where m is mass, w is weight and g is the acceleration induced by gravity, and F=m.multidot.a, where F is force and a is acceleration, then F=(w/g).multidot.a. It is convenient to express acceleration of an object in terms of a multiple of g, i.e. the "G" level. Thus, if an object changes velocity at twice the gravitational acceleration g, it experiences a 2G force and the object will appear to weigh twice as much as it normally does in the g field.
A position insensitive shock sensor is disclosed in U.S. Pat. No. 4,683,355 issued to the present inventor on July 28, 1987 and is herein expressly incorporated by reference. Attached FIG. 1 shows such a shock sensor wherein a mercury wettable insert 234 supports a mercury mass 218 in spaced relationship to a first contact terminal 224. The shock sensor includes an electrically conductive base 220 in contact with an electrically conductive cover plate 212 to form a sealed housing. A mercury wettable tubular insert 234 is positioned inside of base 220. Mercury completely fills a bore of the tubular insert, except for the meniscus formed at each end of the insert 234. Gaskets 232 and 238, made from fibrous felt-like material or mesh that is gas permeable and somewhat mercury impermeable, axially space the insert 234 with respect to the end walls of base 220 and cover plate 212. The first contact terminal 224 is electrically isolated from the end wall of base 220. A second contact terminal 216 is connected to the cover plate 212 which in turn is connected to the base 220 that forms an electrical contact via radial protrusions 235 with the mercury wettable tubular insert 234. Thus, the second contact terminal 216 is in electrical communication with the mercury reservoir 218, so when the mercury makes contact with the first contact terminal 224 an electrical circuit is created. Until then, the mercury is held within the bore of the insert by surface tension.
The surface tension of a liquid is a force that causes a droplet of the liquid to assume a spherical shape in a zero gravity field. However, the shape and position of a body of liquid at rest and under conditions of constant pressure and temperature depend upon the equilibrium of three influences. These are (1) the surface tension of the liquid, (2) the magnitude and direction of all external forces, including gravity, acting on the liquid, and (3) the degree of wetting between the liquid and any solid surface in contact with the liquid.
A surface is considered to be wetted with a liquid if the liquid forms a low contact angle with the surface. A small quantity of liquid on a non-wetted surface will tend to bead, i.e. form a high contact angle, while on a wetted surface, the liquid will tend to spread itself uniformly over the wetted surface. In addition, as a result of a tilt or an impact, the liquid will leave a non-wetted surface without generating any restoring surface tension forces, whereas resisting forces will be present in the case of a liquid wetted to a surface.
When the shock sensor of FIG. 1 is subjected to a shock, the mercury is redistributed to protrude outwardly from the insert 234 to make contact with the first contact terminal 224 and thus close the electric circuit. Since the gaskets 232 and 238 are gas permeable, gas escapes from the protruding mercury, and does not impede the movement of the mercury. When the force of the shock subsides, the surface tension of the mercury causes the mercury to be retracted into the tubular insert 234.
While this mercury shock sensor overcomes many of the deficiencies of the previous switches, room for improvement remains. For example, the closure of the switch occurs virtually instantaneously in response to a shock in excess of a predetermined. value. However, in some cases a selectable closure delay, i.e. a delay in the operation time of the switch, would be desirable such as when the switch is to be introduced into a system in which an impact event or shock is to be distinguished from transitory vibrations. For example, if the switch is to be used in a vehicle air bag safety system, it is desirable to distinguish an actual crash event from the transitory vibration caused by running over potholes, emergency braking, etc. Similarly, if the switch is to be used in an emergency location transmitter (ELT), it would be desirable to distinguish between an actual crash event and transitory vibrations due to engine vibration, air turbulence, for example. If the shock sensor is to be used in a military ordnance fuse, it is highly desirable to distinguish between the impact of the ordnance on its target and the jostling that the ordnance may undergo during transportation. Absent the knowledge of how to create a closure delay in the shock sensor switch itself, additional electronic timing elements, such as disclosed in U.S. Pat. 4,477,732, would be required in order to achieve such a delay.
Further room for improvement in the FIG. 1 sensor remains in connection with a potential loss of mercury during certain switching conditions. In that regard, the mesh or screen of the FIG. 1 mercury switch sensor is mercury impermeable under normal operating conditions such as when the loading force on the mercury is perpendicular to the plane of the mesh, i.e., when the direction of the loading force coincides with the axis of the bore of the insert 234. This is so because the spaces between the woven wires of the mesh are small enough that the capillary repulsion caused by the mercury non-wettable nature of the wire mesh and the surface tension of the mercury prevents the mercury from passing through the mesh. As the mercury is forced against the mesh, the mercury protrudes somewhat into the mesh. The capillary attractive forces of the mercury cause the mercury to "ball" as much as possible. The balling of the mercury causes frictional forces to develop which resist passage of the mercury through the screen.
If the loading force is great enough to overcome the frictional forces, however, the mercury will pass through the mesh and reform into a ball on the other side. The size of the interstices is selected to minimize that occurrence. However, the size of the interstices varies according to the perspective or relative orientation of the load. In the axial direction the mercury "sees" the smallest apparent interstices, so the mesh is virtually mercury impermeable. Under other orientations of the load, such as loads oriented at 45.degree. to the plane of the mesh, the mercury "sees" much larger interstices. Thus, when the shock sensor is subjected to side-loading (i.e. the "G" force developing in a direction perpendicular to the switch axis), the mercury faces relatively larger interstices and may be able to escape through the screen under otherwise normal operating conditions.
Additional considerations involving the FIG. 1 switch involve the fact that the outer casing or base 220 is made of an electrically conductive material and forms part of the overall circuit into which the switch is incorporated. Thus, care must be taken when positioning the switch to ensure that the switch does not short itself or other adjacent circuitry by inadvertently making contact with other conductive surfaces. Further, the requirement that the base 220 be made of an electrically conductive material restricts the types of materials and manufacturing processes which might be used in producing the switch.